LEDs are built on a substrate and are doped with impurities to create a p-n junction. A current flows from the p-side, or anode, to the n-side, or cathode, but not in the reverse direction. Electrons and holes flow into the p-n junction from electrodes with different voltages. If an electron meets a hole, it falls into a lower energy level and releases energy in the form of a photon. The wavelength of the light emitted by the LED and the color of the light may depend on the band gap energy of the materials forming the p-n junction.
FIG. 1 is a cross-sectional side view of a vertical LED structure. The LED 100 has a series of p electrodes (or p mirrors) 101 (illustrated in FIGS. 1-2 by the hash marks). These p electrodes 101 function as a p-ohmic contact and/or an optical reflection mirror. In other words, the p electrodes 101 serve to reflect light back toward the upper surface (i.e. above the n-type layer 105). In addition, these p electrodes serve as the electron source for the LED.
A p-type layer 103 is disposed on the p electrodes 101. A multiple quantum well (MQW) 104 is disposed on the p-type layer 103. An n-type layer 105 is disposed on the MQW 104. Finally, n electrodes 106 are disposed on the n-type layer 105. This LED 100 may be mounted on a metal alloy in one instance. The p-type layer 103 and n-type layer 105 may be, for example, GaN or AlGaInP. The MQW 104 may be GaInN or AlGaInP.
One shortcoming of this configuration is that the current preferably flows directly toward the n electrodes 106, as shown by arrows 109. This means that light is predominantly generated in areas within the LED which are blocked by the n electrodes 106.
To overcome this shortcoming, several methods are used. In some embodiments, disruption strips 102 (illustrated in FIGS. 2A-B by shading) interrupt the p electrodes 101. FIG. 2A is a bottom (or p-side) perspective view of a vertical LED structure and FIG. 2B is a cross-sectional side view of the vertical LED structure in FIG. 1. The LED 100 has a series of p electrodes (or p mirrors) 101 (illustrated in FIGS. 1-2 by the hash marks). These p electrodes 101 function as a p-ohmic contact and/or an optical reflection mirror. The disruption strips 102 force a current to spread over the areas of the LED 100 that are not shadowed by the n electrodes 106, as shown by the arrows 110. Such current spreading may increase the efficiency and brightness of the LED 100. In some embodiment, these disruption strips 102 may be an oxide, which is non-conductive. In other embodiments, these disruption strips may connect to p-type layer 103 through non-ohmic contacts, which increases their resistivity.
Other techniques include coating portions of the lower surface of the p-type layer 103 with a dielectric material to disrupt the current flow. Another technique involves deposition of a dielectric on the lower surface of the p-type layer 103. However, these techniques are known to be susceptible to peeling.
Current spreading in the LED 100 increases the brightness of the LED 100 because any current concentration or light generation under the n electrodes 106 is reduced or eliminated. Furthermore, spreading a current over the entire device area of the LED 100 may increase excitation efficiency. Disruption of the p electrodes 101, however, sacrifices optical reflection from the side of the LED 100 with the p-type layer 103 because the optical reflection will be reduced as the area of the p electrodes 101 is reduced. Thus, reducing the area of the p electrodes 101 will reduce reflection or mirroring. Interrupting the p electrodes 101 may cause a significant brightness loss in one instance.
Vertical LED structures have optical reflection problems. Accordingly, there is a need in the art for an improved vertical LED structure and a method of ion implantation to form an improved vertical LED structure.